Solid electrolyte sheet, method for producing same and all-solid-state secondary battery

ABSTRACT

Provided are a sodium ion-conductive crystal-containing solid electrolyte sheet capable of giving excellent battery characteristics even when reduced in thickness, and an all-solid-state battery using the same. The solid electrolyte sheet contains at least one type of sodium ion-conductive crystal selected from β″-alumina and NASICON crystal and has a thickness of 500 μm or less and a flatness of 200 μm or less.

TECHNICAL FIELD

The present invention relates to sodium ion-conductivecrystal-containing solid electrolyte sheets for use in power storagedevices, such as sodium ion secondary batteries, methods for producingthe same, and sodium ion all-solid-state secondary batteries.

BACKGROUND ART

Lithium ion secondary batteries have secured their place ashigh-capacity and light-weight power sources essential for mobiledevices, electric vehicles, and so on. However, current lithium ionsecondary batteries employ as their electrolytes, mainly, combustibleorganic electrolytic solutions and, therefore, raise concerns about therisk of ignition or the like. As a solution to this problem,developments of lithium ion all-solid-state batteries using a solidelectrolyte instead of an organic electrolytic solution have beenpromoted (see, for example, Patent Literature 1).

However, an issue of concern with lithium is global price increase ofraw and processed materials therefor. To cope with this, sodium hasattracted attention as a material to replace lithium and there isproposed a sodium ion all-solid-state battery in which NASICON-typesodium ion-conductive crystals made of Na₃Zr₂Si₂PO₁₂ are used as a solidelectrolyte (see, for example, Patent Literature 2).

Alternatively, beta-alumina-based solid electrolytes, includingβ-alumina and β″-alumina, are also known to exhibit high sodium-ionconductivity. These solid electrolytes are also used as solidelectrolytes for sodium-sulfur batteries.

In relation to all-solid-state batteries, a smaller thickness of thesolid electrolyte is preferred because the resistance to ion migrationin a battery (for example, a sodium ion all-solid-state battery) becomessmaller and the energy density per unit volume becomes higher.Therefore, there is a demand to reduce the thickness of the solidelectrolyte (produce a sheet-form solid electrolyte).

CITATION LIST Patent Literature

[PTL 1]

-   JP-A-H05-205741    [PTL 2]-   JP-A-2010-15782

SUMMARY OF INVENTION Technical Problem

If the thickness of the solid electrolyte is reduced, the internalresistance of the battery tends to increase, which presents a problemthat the battery characteristics, such as discharge capacity andoperating voltage, decrease.

Therefore, the present invention has an object of providing, as a firstaspect, a sodium ion-conductive crystal-containing solid electrolytesheet capable of giving excellent battery characteristics even whenreduced in thickness, and an all-solid-state battery using the same.

Furthermore, a sheet-form solid electrolyte is produced, for example, bya method (green sheet method) of making a raw material powder for thesolid electrolyte into a slurry, forming a green sheet from the slurry,and then firing the green sheet. However, when a sodium ion-conductivecrystal-containing solid electrolyte sheet is produced by the abovemethod, there arises a problem that its ionic conductivity is likely todecrease. As a result, an all-solid-state battery produced using theabove solid electrolyte sheet tends to have a low discharge capacity.

Therefore, the present invention has another object of providing, as asecond aspect, a sodium ion-conductive crystal-containing solidelectrolyte sheet having a high ionic conductivity, and a sodium ionall-solid-state battery using the same.

Solution to Problem

A solid electrolyte sheet according to a first aspect of the presentinvention contains at least one type of sodium ion-conductive crystalselected from β″-alumina and NASICON crystal and has a thickness of 500μm or less and a flatness of 200 μm or less.

Studies by the inventors have found that the reason why a reduction inthickness of a solid electrolyte leads to increased internal resistanceof a battery is attributable to the flatness of the solid electrolyte.In JIS, the term flatness is defined as “the magnitude of a deviationfrom the geometrically precise plane of a plane form.” FIG. 1 is aschematic cross-sectional view for illustrating the flatness of a solidelectrolyte sheet. As shown in FIG. 1 , the flatness of a solidelectrolyte sheet indicates the magnitude of a gap created when onesurface of the sheet is sandwiched between two parallel planes. If thevalue of the flatness of the solid electrolyte is large, an electrodematerial cannot uniformly be applied to the surface of the solidelectrolyte, so that the electrode has an uneven thickness and thereforeundergoes a local increase in internal resistance. Hence, by making thesolid electrolyte to have a small flatness as described above, thethickness of the electrode can be uniform, so that the internalresistance of the battery can be small. As a result, the batterycharacteristics, such as discharge capacity and operating voltage, canbe increased. Furthermore, when the flatness of the solid electrolyte issmall, the handleability increases, so that the occurrence of cracks andso on during production of the battery can be reduced.

A solid electrolyte sheet according to a second aspect of the presentinvention is a solid electrolyte sheet containing at least one type ofsodium ion-conductive crystal selected from β″-alumina and NASICONcrystal, wherein when C₁ represents a Na₂O concentration at a depth of 5μm from a surface of the solid electrolyte sheet and C₂ represents aNa₂O concentration at a depth of 20 μm from the surface, C₂-C₁≤10% bymole.

Studies by the inventors have found that the ionic conductivity of asolid electrolyte sheet correlates with the Na₂O concentration in thesurface layer of the sheet. Specifically, it has been found that whenthe Na₂O concentration in the sheet surface layer decreases, the ionicconductivity of the solid electrolyte sheet decreases. In view of this,when the Na₂O concentration in the surface layer of the solidelectrolyte sheet is made relatively large (the difference from the Na₂Oconcentration in the inside of the sheet is made small), the ionicconductivity can be increased.

Another solid electrolyte sheet according to the second aspect of thepresent invention is a solid electrolyte sheet containing at least onetype of sodium ion-conductive crystal selected from β″-alumina andNASICON crystal, wherein when, with a thickness of the solid electrolytesheet represented as 100%, C₁′ represents a Na₂O concentration at adepth of 5% from a surface of the solid electrolyte sheet and C₂′represents a Na₂O concentration at a depth of 50% from the surface,C₂′−C₁′≤10% by mole.

The solid electrolyte sheet according to the second aspect of thepresent invention preferably has a thickness of 500 μm or less. When thesolid electrolyte sheet is made thin as just described, it is preferredbecause the resistance to ion migration in the resultant batterydecreases and the energy density per volume increases.

The solid electrolyte sheets according to the first and second aspectsof the present invention preferably contain, in terms of % by mole, 65to 98% Al₂O₃, 2 to 20% Na₂O, 0.3 to 15% MgO+Li₂O, 0 to 20% ZrO₂, and 0to 5% Y₂O₃. Note that “(component)+(component)+ . . . ” as used hereinmeans the total sum of the contents of the mentioned components.

The solid electrolyte sheets according to the first and second aspectsof the present invention preferably contain crystals represented by ageneral formula Na_(s)A1_(t)A2_(u)O_(v) (where A1 is at least oneselected from Al, Y, Yb, Nd, Nb, Ti, Hf, and Zr, A2 is at least oneselected from Si and P, s=1.4 to 5.2, t=1 to 2.9, u=2.8 to 4.1, and v=9to 14).

The solid electrolyte sheets according to the first and second aspectsof the present invention are suitable for all-solid-state sodium ionsecondary batteries.

An all-solid-state secondary battery according to the present inventionis an all-solid-state battery including a positive electrode compositelayer, a solid electrolyte layer, and a negative electrode compositelayer, wherein the solid electrolyte layer is formed of theabove-described solid electrolyte sheet according to the first or secondaspect.

A method for producing a solid electrolyte sheet according to thepresent invention is a method for producing the above-described solidelectrolyte sheet according to the first or second aspect and includesthe steps of: (a) pre-firing a raw material powder; (b) making thepre-fired raw material powder into a slurry; (c) applying the slurry ona support and drying the slurry to obtain a green sheet; and (d) firingthe green sheet to form sodium ion-conductive crystals.

When a solid electrolyte sheet is produced by the green sheet method,there arises a problem that the flatness of the solid electrolyte sheetis deteriorated by contraction during firing. To cope with this, a rawmaterial powder is previously pre-fired to produce a composite oxide(for example, β-alumina) as a precursor and the composite oxide isconverted to sodium ion-conductive crystals by later firing (forexample, β-alumina is changed in phase to β″-alumina). Thus, thecontraction during firing can be reduced and the deterioration inflatness of the solid electrolyte sheet can be therefore reduced.

Furthermore, particularly when the thickness of the green sheet is madesmall, cracks may be formed in the sheet during firing. This can beattributed to the release of carbon dioxide from a carbonate rawmaterial and volatilization of a sodium component and so on duringfiring. When the raw material powder is pre-fired to previously causethe release of carbon dioxide and the volatilization of a sodiumcomponent and so on, the release of carbon dioxide and thevolatilization of the sodium component and so on during the later firingcan be reduced, so that problems of the deterioration in flatness of thesolid electrolyte sheet and the formation of cracks in the solidelectrolyte sheet can be reduced.

In addition, studies by the inventors have found that, during firing ofa green sheet in the production of a solid electrolyte sheet, a sodiumcomponent volatilizes from the surface of the green sheet, so that theNa₂O concentration in the surface layer of the solid electrolyte sheetdecreases and the ionic conductivity therefore decreases. In thisregard, it can be considered that when the sodium component volatilizesfrom the surface of the green sheet, a sodium-free other crystal layer(for example, a MgAl₂O₄ layer) is formed on the surface layer of thesolid electrolyte sheet, so that the ionic conductivity decreases. Theother crystal layer has a problem that particularly when the thicknessof the solid electrolyte sheet is small, an attempt to remove the othercrystal layer by polishing is likely to cause cracking and chipping ofthe sheet. Unlike this, when the raw material powder is previouslypre-fired, the volatilization of the sodium component and so on duringthe later firing can be reduced, so that the ionic conductivity of thesolid electrolyte sheet can be increased. This can be attributed to thefact that the raw material powder is pre-fired to produce a compositeoxide (for example, β-alumina) as a precursor, the composite oxide isconverted to sodium ion-conductive crystals by the later firing (forexample, β-alumina is changed in phase to β″-alumina), and the sodiumcomponent in the composite oxide is difficult to volatilize during thefiring.

In the method for producing a solid electrolyte sheet according to thepresent invention, the green sheet is preferably fired on an MgO setter.If during firing of the green sheet the raw material powder reacts witha setter, the Na₂O concentration in the surface layer of the solidelectrolyte sheet may decrease. However, an MgO setter has lowreactivity particularly with a raw material powder for use in producinga solid electrolyte sheet containing β″-alumina. Therefore, the reactionof the raw material powder with the setter during firing of a greensheet can be reduced, so that the decrease in Na₂O concentration in thesurface layer of the solid electrolyte sheet can be reduced.

In the step (d) in the method for producing a solid electrolyte sheetaccording to the present invention, the firing is preferably performedin a state where the green sheet is placed between an upper setter and alower setter and a clearance is provided between the green sheet and theupper setter. In this case, the clearance is preferably 1 to 500 μm. Bydoing so, the deterioration in flatness and the formation of cracks dueto contraction of the green sheet during firing can be further reduced.Specifically, since the green sheet is sandwiched between the setters,the occurrence of undulation due to contraction of the green sheetduring firing can be reduced. Since a slight clearance is providedbetween the green sheet and the upper setter, the green sheet canmoderately contract without being excessively constrained by the settersduring firing, so that strain is less likely to occur and the formationof cracks can be reduced.

Advantageous Effects of Invention

The present invention enables provision of a sodium ion-conductivecrystal-containing solid electrolyte sheet capable of giving excellentbattery characteristics even when reduced in thickness, and anall-solid-state battery using the same. Furthermore, the presentinvention enables provision of a sodium ion-conductivecrystal-containing solid electrolyte sheet having a high ionicconductivity, and a sodium ion all-solid-state battery using the same.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view for illustrating the flatnessof a solid electrolyte sheet.

FIG. 2 is a graph showing, in an example of a solid electrolyte sheetaccording to a first aspect, a powder X-ray diffraction pattern of a rawmaterial powder of the solid electrolyte sheet.

FIG. 3 is a graph showing, in the example of the solid electrolyte sheetaccording to the first aspect, a powder X-ray diffraction pattern of theraw material powder after being pre-fired.

FIG. 4 is a graph showing, in the example of the solid electrolyte sheetaccording to the first aspect, a powder x-ray diffraction pattern of theobtained solid electrolyte sheet.

FIG. 5 is a graph showing, in an example of a solid electrolyte sheetaccording to a second aspect, a powder X-ray diffraction pattern of araw material powder of the solid electrolyte sheet.

FIG. 6 is a graph showing, in the example of the solid electrolyte sheetaccording to the second aspect, a powder X-ray diffraction pattern ofthe raw material powder after being pre-fired.

FIG. 7 is a graph showing, in the example of the solid electrolyte sheetaccording to the second aspect, a powder X-ray diffraction pattern ofthe obtained solid electrolyte sheet.

DESCRIPTION OF EMBODIMENTS

Solid electrolyte sheets according to first and second aspects of thepresent invention contain at least one type of sodium ion-conductivecrystal selected from β″-alumina and NASICON crystal. β″-alumina andNASICON crystal are preferred because they have excellent sodium-ionconductivity, high electron insulating properties, and excellentstability.

The solid electrolyte sheet according to the first aspect of the presentinvention is characterized by containing at least one type of sodiumion-conductive crystal selected from β″-alumina and NASICON crystal andhaving a thickness of 500 μm or less and a flatness of 200 μm or less.

A smaller thickness of the solid electrolyte sheet is more preferredbecause the distance taken to conduct ions in the solid electrolytebecomes shorter to increase the ionic conductivity. Furthermore, withthe use of the solid electrolyte sheet as a solid electrolyte for anall-solid-state battery, the all-solid-state battery has a higher energydensity per unit volume. Specifically, the thickness of the solidelectrolyte sheet according to the first aspect of the present inventionis 500 μm or less, preferably 400 μm or less, more preferably 300 μm orless, and particularly preferably 200 μm or less. However, if thethickness of the solid electrolyte sheet is too small, the mechanicalstrength may decrease and the positive and negative electrodes may beshort-circuited. Therefore, the thickness is preferably not less than 5μm, more preferably not less than 10 μm, still more preferably not lessthan 20 μm, yet still more preferably not less than 30 μm, andparticularly preferably not less than 50 μm.

The flatness of the solid electrolyte sheet according to the firstaspect of the present invention is 200 μm or less, preferably 150 μm orless, more preferably 100 μm or less, and particularly preferably 50 μmor less. If the value of flatness is too large, an electrode materialcannot uniformly be applied to the surface of the solid electrolyte, sothat the electrode has an uneven thickness and therefore undergoes alocal increase in internal resistance. As a result, the batterycharacteristics, such as discharge capacity, operating voltage, and,furthermore, rate characteristic, tend to decrease. The lower limit ofthe value of flatness is not particularly limited, but it is, actually,preferably not less than 1 μm and more preferably not less than 5 μm.

The solid electrolyte sheet according to the second aspect of thepresent invention is characterized in that when C₁ represents a Na₂Oconcentration at a depth of 5 μm from a surface of the solid electrolytesheet and C₂ represents a Na₂O concentration at a depth of 20 μm fromthe surface, C₂-C_(1≤)10% by mole. If C₂−C₁ is too large, the ionicconductivity of the solid electrolyte sheet is likely to decrease.Therefore, C₂−C₁ is preferably 8% by mole or less, more preferably 6% bymole or less, and particularly preferably 3% by mole or less.

As another point of view, the solid electrolyte sheet according to thesecond aspect of the present invention is characterized in that when,with the thickness of the solid electrolyte sheet represented as 100%,C₁′ represents a Na₂O concentration at a depth of 5% from a surface ofthe solid electrolyte sheet and C₂′ represents a Na₂O concentration at adepth of 50% from the surface, C₂′−C₁′≤10% by mole. If C₂′−C₁′ is toolarge, the ionic conductivity of the solid electrolyte sheet is likelyto decrease. Therefore, C₂′−C₁′ is preferably 8% by mole or less, morepreferably 6% by mole or less, and particularly preferably 3% by mole orless.

A smaller thickness of the solid electrolyte sheet is more preferredbecause the distance taken to conduct ions in the solid electrolytebecomes shorter to increase the ionic conductivity. Furthermore, withthe use of the solid electrolyte sheet as a solid electrolyte for anall-solid-state battery, the all-solid-state battery has a higher energydensity per unit volume. Specifically, the thickness of the solidelectrolyte sheet according to the second aspect of the presentinvention is preferably 500 μm or less, more preferably 400 μm or less,still more preferably 300 μm or less, and particularly preferably 200 μmor less. However, if the thickness of the solid electrolyte sheet is toosmall, the mechanical strength may decrease and the positive andnegative electrodes may be short-circuited. Therefore, the thickness ispreferably not less than 5 μm, more preferably not less than 10 μm,still more preferably not less than 20 μm, yet still more preferably notless than 30 μm, and particularly preferably not less than 50 μm.

As the solid electrolyte sheet is thinner, the difference in Na₂Oconcentration between the surface layer and the inside of the sheetduring firing of a green sheet tends to increase. Therefore, when thesolid electrolyte sheet is thin, the effect obtained by applying theproduction method according to the present invention can be more likelyto be given. The reasons why as the solid electrolyte sheet is thinner,the difference in Na₂O concentration between the surface layer and theinside of the sheet during firing of a green sheet tends to increase canbe considered as follows. In the case where the solid electrolyte sheetis thick, a large amount of sodium component exists in the inside of thesheet. Therefore, even when the sodium component volatilizes from thesheet surface layer during firing of a green sheet, the sodium componentis supplied to the surface layer and, therefore, the difference in Na₂Oconcentration between the surface layer and the inside of the sheet isless likely to become large. On the other hand, in the case where thesolid electrolyte sheet is thin, a small amount of sodium componentexists in the inside of the sheet. Therefore, when the sodium componentvolatilizes from the sheet surface layer during firing of a green sheet,the sodium component is less likely to be supplied from the inside tothe surface layer of the sheet and, therefore, the difference in Na₂Oconcentration between the surface layer and the inside of the sheet islikely to become large.

The following description relates to the structure common to the solidelectrolyte sheets according to the first and second aspects of thepresent invention, unless otherwise stated.

Specific examples of β″-alumina include the following trigonal crystals:(Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O), (Al_(8.87)Mg_(2.13)O₁₆)(Na_(3.13)O), Na_(1.67)Mg_(0.67)Al_(10.33)O₁₇,Na_(1.49)Li_(0.25)Al_(10.75)O₁₇, Na_(1.72)Li_(0.3)Al_(10.66)O₁₇, andNa_(1.6)Li_(0.34)Al_(10.66)O₁₇. The solid electrolyte sheet may contain,in addition to β″-alumina, β-alumina. Examples of β-alumina include thefollowing hexagonal crystals: (Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O),(Al_(10.37)Mg_(0.63)O₁₆) (Na_(1.63)O), NaAl₁₁O₁₇, and(Al_(10.32)Mg_(0.68)O₁₆) (Na_(1.68)O).

An example of a specific composition of the solid electrolyte sheetaccording to the present invention containing β″-alumina is acomposition containing, in terms of % by mole, 65 to 98% Al₂O₃, 2 to 20%Na₂O, 0.3 to 15% MgO+Li₂O, 0 to 20% ZrO₂, and 0 to 5% Y₂O₃. Reasons whythe composition is limited as just described will be described below.

Al₂O₃ is a main component that forms β″-alumina. The content of Al₂O₃ ispreferably 65 to 98% and particularly preferably 70 to 95%. If Al₂O₃ istoo less, the ionic conductivity of the solid electrolyte is likely todecrease. On the other hand, if Al₂O₃ is too much, α-alumina having nosodium-ion conductivity remains in the solid electrolyte, so that theionic conductivity of the solid electrolyte is likely to decrease.

Na₂O is a component that gives the solid electrolyte sodium-ionconductivity. The content of Na₂O is preferably 2 to 20%, morepreferably 3 to 18%, and particularly preferably 4 to 16%. If Na₂O istoo less, the above effect is less likely to be achieved. On the otherhand, if Na₂O is too much, surplus sodium forms compounds notcontributing to ionic conductivity, such as NaAlO₂, so that the ionicconductivity is likely to decrease.

MgO and Li₂O are components (stabilizing agents) that stabilize thestructure of β″-alumina. The content of MgO+Li₂O is preferably 0.3 to15%, more preferably 0.5 to 10%, and particularly preferably 0.8 to 8%.If MgO+Li₂O is too less, α-alumina remains in the solid electrolyte, sothat the ionic conductivity is likely to decrease. On the other hand, ifMgO+Li₂O is too much, MgO or Li₂O having failed to function as astabilizing agent remains in the solid electrolyte, so that the ionicconductivity is likely to decrease.

ZrO₂ and Y₂O₃ have the effect of inhibiting abnormal grain growth ofβ″-alumina during firing to increase the adhesion of particles ofβ″-alumina. As a result, the ionic conductivity of the solid electrolytesheet is likely to increase. In relation to the solid electrolyte sheetaccording to the second aspect, when the adhesion between particles ofβ″-alumina increases, the sodium component is less likely to volatilizefrom the surface of a green sheet during firing of the green sheet, sothat the Na₂O concentration in the surface layer of the solidelectrolyte sheet can be increased. The content of ZrO₂ is preferably 0to 15%, more preferably 1 to 13%, and particularly preferably 2 to 10%.The content of Y₂O₃ is preferably 0 to 5%, more preferably 0.01 to 4%,and particularly preferably 0.02 to 3%. If ZrO₂ or Y₂O₃ is too much, theamount of β″-alumina produced decreases, so that the ionic conductivityof the solid electrolyte is likely to decrease.

The NASICON crystals are preferably made of a compound represented by ageneral formula Na_(s)A1_(t)A2_(u)O_(v) (where A1 is at least oneselected from Al, Y, Yb, Nd, Nb, Ti, Hf, and Zr, A2 is at least oneselected from Si and P, s=1.4 to 5.2, t=1 to 2.9, u=2.8 to 4.1, and v=9to 14). In this relation, A1 is preferably at least one selected from Y,Nb, Ti, and Zr. By doing so, crystals having excellent ionicconductivity can be obtained.

The respective preferred ranges of the indices in the above generalformula are as follows.

The index s is preferably 1.4 to 5.2, more preferably 2.5 to 3.5, andparticularly preferably 2.8 to 3.1. If s is too small, the amount ofsodium ions is small, so that the ionic conductivity is likely todecrease. On the other hand, if s is too large, surplus sodium formscompounds not contributing to ionic conductivity, such as sodiumphosphate and sodium silicate, so that the ionic conductivity is likelyto decrease.

The index t is preferably 1 to 2.9, more preferably 1 to 2.5, andparticularly preferably 1.3 to 2. If t is too small, thethree-dimensional network in crystals reduces, so that the ionicconductivity is likely to decrease. On the other hand, if t is toolarge, compounds not contributing to ionic conductivity, such aszirconia and alumina, are formed, so that the ionic conductivity islikely to decrease.

The index u is preferably 2.8 to 4.1, more preferably 2.8 to 4, stillmore preferably 2.9 to 3.2, and particularly preferably 2.95 to 3.1. Ifu is too small, the three-dimensional network in crystals reduces, sothat the ionic conductivity is likely to decrease. On the other hand, ifu is too large, crystals not contributing to ionic conductivity areformed, so that the ionic conductivity is likely to decrease.

The index v is preferably 9 to 14, more preferably 9.5 to 12, andparticularly preferably 11 to 12. If v is too small, Al (for example, analuminum component) has a low valence, so that the electric insulationproperty is likely to decrease. On the other hand, if v is too large, aperoxidative state occurs, so that sodium ions are attracted bylonepairs of electrons of oxygen atoms and, therefore, the ionicconductivity is likely to decrease.

The above-described NASICON crystals are preferably monoclinic crystals,hexagonal crystals or trigonal crystals, and particularly preferablymonoclinic or trigonal because they have excellent ionic conductivity.

Specific examples of the NASICON crystal include the following crystals:Na₃Zr₂Si₂PO₁₂, Na_(3.2)Zr_(1.3)Si_(2.2)P_(0.8)O_(10.5),Na₃Zr_(1.6)Ti_(0.4)Si₂PO₁₂, Na₃Hf₂Si₂PO₁₂,Na_(3.4)Zr_(0.9)Hf_(1.4)Al_(0.6)Si_(1.2)P_(1.8)O₁₂,Na₃Zr_(1.7)Nb_(0.24)Si₂PO₁₂, Na_(3.6)Ti_(0.2)Y_(0.8)Si_(2.8)O₉,Na₃Zr_(1.88)Y_(0.12)Si₂PO₁₂, Na_(3.12)Zr_(1.88)Y_(0.12)Si₂PO₁₂,Na_(3.6)Zr_(0.13)Yb_(1.67)Si_(0.11)P_(2.9)O₁₂, and Na₅YSi₄O₁₂.Particularly, Na_(3.12)Zr_(1.88)Y_(0.12)Si₂PO₁₂ is preferred because ithas excellent ionic conductivity.

Next, a description will be given of a method for manufacturing thesolid electrolyte sheets according to the first and second aspects ofthe present invention. The solid electrolyte sheets according to thefirst and second aspects of the present invention can be produced by amethod including the steps of: (a) pre-firing a raw material powder; (b)making the pre-fired raw material powder into a slurry; (c) applying theslurry on a support and drying the slurry to obtain a green sheet; and(d) firing the green sheet to form sodium ion-conductive crystals.

In the case where the solid electrolyte sheet contains β″-alumina, theraw material powder contains Al₂O₃ as a main component. Specifically,the raw material powder preferably contains, in terms of % by mole, 65to 98% Al₂O₃, 2 to 20% Na₂O, 0.3 to 15% MgO+Li₂O, 0 to 20% ZrO₂, and 0to 5% Y₂O₃. Because reasons why the composition is limited as justdescribed are as described previously, further explanation will beomitted.

In the case where the solid electrolyte sheet contains NASICON crystals,the raw material powder preferably contains, in terms of % by mole, 17.5to 50% Na₂O, 12 to 45% Al₂O₃+Y₂O₃+Yb₂O₃+Nd₂O₃+Nb₂O₅+TiO₂+HfO₂+ZrO₂, and24 to 54% SiO₂+P₂O₅. When the composition is limited as above, desiredNASICON crystals can be precipitated.

The average particle diameter (D₅₀) of the raw material powder ispreferably 10 μm or less. If the average particle diameter of the rawmaterial powder is too large, the contact area between the raw materialpowder particles decreases, so that a solid-phase reaction is lesslikely to sufficiently progress. Furthermore, the solid electrolytesheet tends to be difficult to reduce in thickness. The lower limit ofthe average particle diameter of the raw material powder is notparticularly limited, but it is, actually, not less than 0.1 m.

In producing the solid electrolyte sheet according to the first aspect,the raw material powder for the solid electrolyte sheet is pre-fired toproduce a composite oxide (for example, β-alumina) as a precursor andthe composite oxide is converted to sodium ion-conductive crystals bylater firing (for example, β-alumina is changed in phase to β″-alumina).Thus, the contraction during the firing can be reduced and thedeterioration in flatness of the solid electrolyte sheet can betherefore reduced. Furthermore, when the raw material powder ispreviously pre-fired, the release of carbon dioxide from a carbonate rawmaterial and the volatilization of a sodium component and so on duringthe later firing can be reduced, so that the volume contraction of thegreen sheet can be reduced and the deterioration in flatness of thesolid electrolyte sheet and the formation of cracks in the solidelectrolyte sheet can be therefore reduced.

In producing the solid electrolyte sheet according to the second aspect,the raw material powder is pre-fired to produce a composite oxide (forexample, β-alumina) as a precursor and the composite oxide is convertedto sodium ion-conductive crystals by later firing (for example,β-alumina is changed in phase to β″-alumina). By doing so, thevolatilization of a sodium component and so on during the firing can bereduced and, for the above-described reason, the ionic conductivity ofthe solid electrolyte sheet can be increased. Furthermore, since therelease of carbon dioxide from a carbonate raw material is caused by thepre-firing, the release of carbon dioxide during the firing is reduced,so that a dense sheet can be produced.

In the case where the solid electrolyte sheet contains β″-alumina, thepre-firing temperature is preferably 1000° C. to below 1400° C., morepreferably 1100 to 1350° C., and particularly preferably 1200 to 1300°C. In the case where the solid electrolyte sheet contains NASICONcrystals, the pre-firing temperature is preferably 900° C. to below1200° C., more preferably 1000 to 1180° C., and particularly preferably1050 to 1160° C. If the pre-firing temperature is too low, the aboveeffects are less likely to be achieved. On the other hand, if thepre-firing temperature is too high, the sheet is less likely to besintered during later firing, so that the resultant solid electrolytesheet is less likely to become dense.

The pre-firing time is appropriately adjusted so that the above effectscan be achieved. Specifically, the pre-firing time is preferably 1 to 20hours, more preferably 2 to 18 hours, still more preferably 2 to 15hours, yet still more preferably 2 to 10 hours, and particularlypreferably 3 to 8 hours. If the raw material powder is agglomerated bythe pre-firing, the agglomerates are preferably ground so that the rawmaterial powder has a desired particle diameter.

By the pre-firing, β″-alumina may be produced in addition to β-alumina.Alternatively, a composite oxide other than the above may be produced.Examples of such a composite oxide include those as described below.

Examples of a hexagonal composite oxide include NaAl₅O₈, NaAl₇O₁₁,NaAl_(5.9)O_(9.4), Na₂Al₂₂O₃₄, Na_(2.58)Al_(21.81)O₃₄, NaAl₂₃O₃₅,Na₂Al₂₂O₃₃, Na_(1.5)Al_(10.83)O₁₇, Na_(1.22)Al₁₁O_(17.11),Na_(2.74)Al₂₂O₃₈, Na₂Li_(0.35)Al₁₂₂O_(19.475),Na_(0.45)Li_(0.57)Al₁₁O₁₇, Na_(0.47)Li_(0.75)Al₁₁O_(17.11),NaMg₂Al₁₅O₂₅, Na₂MgAl₁₀O₁₇, Na₂Mg₄Al₃₀O₅₀, andNa_(0.47)Mg_(0.75)Al₁₁O_(17.11).

Examples of a trigonal composite oxide include Na_(1.77)Al₁₁O₁₇,Na_(1.71)Al₁₁O₁₇, and Na₂MgAl₁₀O₁₇.

An example of a tetragonal composite oxide is Na₂Al₂O₄.

An example of a cubic composite oxide is Na₂Al₂O₄.

Examples of an orthorhombic composite oxide include NaA₆O_(9.5),Na_(0.67)Al₆O_(9.33), and Na₅Al₂O₄.

Examples of a monoclinic composite oxide include Na₁₇Al₅O₁₆ andNa₁₄Al₄O₁₃.

An example of a triclinic composite oxide is Na₇Al₃O₈.

In the case where the solid electrolyte sheet contains NASICON crystals,examples of a composite oxide as its precursor include those asdescribed below.

Examples of a cubic composite oxide include Na₂ZrO₃,(ZrO₂)_(0.92)(Na₂O)_(0.04), (ZrO₂)_(0.95)(Na₂O)_(0.025), andNa_(2.47)Zr_(0.13)PO₄.

An example of a hexagonal composite oxide is Na₂ZrO₃.

Examples of a monoclinic composite oxide include Na₆Si₈O₁₉, Na₆Si₂O₇,Na₆Si₈O₁₉, Na₂Si₃O₇, Na₂Si(Si₃O₉), Na₂ZrO₃, and NaZr₅(PO₄)₇.

Examples of an orthorhombic composite oxide include (NaPO₃)₄, Na₃P₃O₉,Na₂Si₄O₉, and Na₁₄Zr₂Si₁₀O₃₁.

An example of a triclinic composite oxide is Na₄SiO₄.

Examples of a trigonal composite oxide include Na_(1.3)Zr_(1.832)(PO₄)₃, Na₈Si(Si₆O₁₈), NaZr₂(PO₄)₃, Na₅Zr(PO₄)₃, and NaZr_(1.88)(PO₄)₃.

Other composite oxides include Na₄P₂O₇, Na(PO₃)₃, NaPO₃, Na₃PO₄,Na₅P₃O₁₀, Na₄P₂O₇, (NaPO₃)₆, Na₄P₂O₆, Na₂SiO₃, Na₂Si₄O₉, Na₂Si₃O₇,Na₂Si₂O₅, Na₂ZrSiO₅, Na₁₄Zr₂Si₁₀O₃₁, and Na_(2.8)Zr_(6.5)Si₂O_(17.8).

A binder, a plasticizer, a solvent, and so on are added to the pre-firedraw material powder and the mixture is kneaded into a slurry.

The solvent may be water or an organic solvent, such as ethanol oracetone. However, when water is used as the solvent, a sodium componentmay elute off from the raw material powder to increase the pH of theslurry and agglomerate the raw material powder. Therefore, an organicsolvent is preferably used.

Next, the obtained slurry is applied onto a support made of PET(polyethylene terephthalate) or so on and dried, thus obtaining a greensheet. The application of the slurry can be implemented with a doctorblade, a die coater or other means. The thickness of the green sheet ispreferably 0.01 to 1 mm, more preferably 0.02 to 1 mm, and particularlypreferably 0.05 to 0.9 mm. If the thickness of the green sheet is toosmall, the mechanical strength of the resultant solid electrolyte sheetmay decrease or the positive and negative electrodes may beshort-circuited. On the other hand, if the thickness of the green sheetis too large, the thickness of the resultant solid electrolyte sheetbecomes large to increase the distance taken to conduct ions in thesolid electrolyte sheet and make the energy density per unit cell likelyto decrease.

Then, the green sheet is fired to produce β″-alumina, thus obtaining asolid electrolyte sheet. Specifically, by the firing, β-alumina ischanged in phase to β″-alumina having excellent ionic conductivity.Alternatively, the green sheet is fired to produce NASICON crystals,thus obtaining a solid electrolyte sheet.

In the case where the solid electrolyte sheet contains β″-alumina, thefiring temperature is preferably 1400° C. or higher, more preferably1450° C. or higher, and particularly preferably 1500° C. or higher. Ifthe firing temperature is too low, the sintering of particles ofβ″-alumina in the solid electrolyte sheet becomes insufficient, so thata dense sheet is less likely to be provided and the ionic conductivityis therefore likely to decrease. Furthermore, the phase change fromβ-alumina to β″-alumina is less likely to occur, so that the ionicconductivity is likely to decrease. On the other hand, the upper limitof the firing temperature is preferably not higher than 1750° C. andparticularly not higher than 1700° C. If the firing temperature is toohigh, the amount of evaporation of sodium component or the like becomeslarge, so that other crystals tend to precipitate and the densenesstends to decrease. As a result, the ionic conductivity of the solidelectrolyte sheet is likely to decrease. The firing time isappropriately adjusted so that produced β″-alumina can be sufficientlysintered. Specifically, the firing time is preferably 10 to 120 minutesand particularly preferably 20 to 80 minutes.

In the case where the solid electrolyte sheet contains NASICON crystals,the firing temperature is preferably 1200° C. or higher and particularlypreferably 1210° C. or higher. If the firing temperature is too low, adense solid electrolyte sheet is less likely to be provided and theionic conductivity is therefore likely to decrease. Furthermore, NASICONcrystals are less likely to precipitate, so that the ionic conductivityis likely to decrease. On the other hand, the upper limit of the firingtemperature is preferably not higher than 1400° C. and particularly nothigher than 1300° C. If the firing temperature is too high, the amountof evaporation of sodium component or the like becomes large, so thatother crystals tend to precipitate and the denseness tends to decrease.As a result, the ionic conductivity of the solid electrolyte sheet islikely to decrease. The firing time is appropriately adjusted so that adense sintered body can be obtained.

When the green sheet is subjected to pressing, such as isostaticpressing, before being fired, the adhesion of particles of β″-alumina orNASICON crystals in the solid electrolyte sheet after being firedincreases, so that the ionic conductivity is likely to increase. In theproduction of a solid electrolyte sheet according to the second aspect,when the adhesion between particles of β″-alumina increases, the sodiumcomponent is less likely to volatilize from the surface of the greensheet during firing of the green sheet, so that the Na₂O concentrationin the surface layer of the solid electrolyte sheet can be increased.

The firing is preferably performed with the green sheet placed on an MgOsetter. By doing so, the reaction of the raw material powder with thesetter during firing of the green sheet can be reduced. As a result, forexample, in producing a solid electrolyte sheet according to the secondaspect, the effect of reducing the decrease in Na₂O concentration in thesurface layer of the solid electrolyte sheet can be obtained. Thiseffect is likely to be obtained particularly in producing a solidelectrolyte sheet containing β″-alumina.

Furthermore, the firing is preferably performed in a state where thegreen sheet is placed between an upper setter and a lower setter and aclearance is provided between the green sheet and the upper setter. Bydoing so, the deterioration in flatness and the formation of cracks dueto contraction of the green sheet during firing can be further reduced.The clearance between the green sheet and the upper setter is preferably1 to 500 μm, more preferably 2 to 400 μm, and particularly preferably 5to 300 μm. If the clearance is too small, the above effects are lesslikely to be achieved. On the other hand, if the clearance is too large,undulation due to contraction of the green sheet during firing occurs,so that the value of flatness of the solid electrolyte sheet tends to belarge.

The solid electrolyte sheet according to the present invention issuitable for use in a sodium ion all-solid-state secondary battery. Thesodium ion all-solid-state secondary battery is made up by including apositive electrode layer formed on one surface of the solid electrolytesheet according to the present invention and a negative electrode layerformed on the other surface of the solid electrolyte sheet. The positiveelectrode layer and the negative electrode layer each contain an activematerial. The active material acts as a positive-electrode activematerial or a negative-electrode active material and can absorb andrelease sodium ions during charge and discharge.

Examples of the positive-electrode active material include: layeredsodium transition metal oxide crystals, such as NaCrO₂, Na_(0.7)MnO₂,and NaFe_(0.2)Mn_(0.4)Ni_(0.4)O₂; sodium transition metal phosphatecrystals containing Na, M (where M represents at least one transitionmetal element selected from Cr, Fe, Mn, Co, and Ni), P, and O, such asNa₂FeP₂O₇, NaFePO₄, and Na₃V₂(PO₄)₃; and like active material crystals.

Particularly, the crystals containing Na, M, P, and O are preferredbecause they have high capacity and excellent chemical stability.Preferred among them are triclinic crystals belonging to space group P1or P-1 and particularly preferred are crystals represented by a generalformula Na_(x)MyP₂O₇ (where 1.20≤x≤2.80 and 0.95≤y≤1.60), because thesecrystals have excellent cycle characteristics.

Examples of the negative-electrode active material include: crystalscontaining at least one selected from Nb and Ti and O, metallic crystalsof at least one selected from Sn, Bi, and Sb; and other active materialcrystals.

The crystals containing at least one selected from Nb and Ti, and O arepreferred because they have excellent cycle characteristics. If thecrystal containing at least one selected from Nb and Ti and O furthercontains Na and/or Li, this is preferred because the charge/dischargeefficiency (the proportion of discharge capacity to charge capacity)increases and a high charge/discharge capacity can be thus maintained.

Above all, if the crystal containing at least one selected from Nb andTi and O is an orthorhombic, hexagonal, cubic or monoclinic crystal,particularly a monoclinic crystal belonging to space group P21/m, thisis preferred because a capacity decrease is less likely to occur evenduring charge and discharge at a large current. An example of theorthorhombic crystal is NaTi₂O₄, examples of the hexagonal crystalinclude Na₂TiO₃, NaTi₈O₁₃, NaTiO₂, LiNbO₃, LiNbO₂, Li₇NbO₆, LiNbO₂, andLi₂Ti₃O₇, examples of the cubic crystal include Na₂TiO₃, NaNbO₃,Li₄Ti₅O₁₂, and Li₃NbO₄, examples of the monoclinic crystal includeNa₂Ti₆O₁₃, NaTi₂O₄, Na₂TiO₃, Na₄Ti₅O₁₂, Na₂Ti₄O₉, Na₂Ti₉O₁₉, Na₂Ti₃O₇,Na₂Ti₃O₇, Li_(1.7)Nb₂O₅, Li_(1.9)Nb₂O₅, Li₁₂Nb₁₃O₃₃, and LiNb₃O₈, and anexample of the monoclinic crystal belonging to space group P21/m isNa₂Ti₃O₇.

The crystal containing at least one selected from Nb and Ti and Opreferably further contains at least one selected from B, Si, P, and Ge.These components have the effect of facilitating the formation of anamorphous phase together with the active material crystals andincreasing the sodium-ion conductivity.

Other negative-electrode active materials that can be used includemetallic crystals of at least one selected from Sn, Bi, and Sb andglasses containing at least one selected from Sn, Bi, and Sb. Thesematerials are preferred because they have high capacity and they areless likely to cause a capacity decrease even during charge anddischarge at a large current.

The positive electrode layer and the negative electrode layer may beelectrode composite layers made of a composite of an active material anda solid electrolyte. The solid electrolyte acts as a sodiumion-conducting path in the electrode composite and can thereforeincrease the discharge capacity and voltage of the battery.

The solid electrolyte that can be used is one obtained by processing theabove-described solid electrolyte sheet into powdered form.

The positive electrode layer and the negative electrode layer preferablyfurther contain a conductive agent.

The conductive agent is a component to be added to the electrode layerin order to achieve a capacity increase and high-rate charge anddischarge of the electrode. Specific examples of the conductive agentinclude highly electrically conductive carbon blacks, such as acetyleneblack and Ketjenblack, graphite, coke, and metal powders, such as Nipowder, Cu powder, and Ag powder. Among them, any of highly electricallyconductive carbon blacks, Ni powder, and Cu powder is preferably used,which exhibit excellent electrical conductivity even when added in verysmall amount.

EXAMPLES

Hereinafter, a description will be given in detail of the presentinvention with reference to its examples, but the present invention isnot limited to these examples.

Solid Electrolyte Sheet According to First Aspect

Table 1 shows Examples 1 to 6 and Table 2 shows Comparative Examples 1to 5.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Pre-firing of Raw Materialdone done done done done done Powder Solid Electrolyte Sheet A A A B A BSolid Electrolyte Sheet 64 114 192 68 71 70 Thickness [μm] SolidElectrolyte Sheet 18 52 148 23 23 27 Flatness [μm] Solid ElectrolytePowder A A A A B B Average Voltage [V] 2.8 2.8 2.7 2.6 2.6 2.5 DischargeCapacity [mAh/g] 73 71 70 65 68 61 Rapid Charge/Discharge 82 77 69 73 7564 Char. [%] Volume Energy Density 82.3 57.1 37.4 65.9 67.4 46.9[mWh/cm³]

TABLE 2 Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5Pre-firing of Raw no no no no no Material Powder Solid Electrolyte SheetA A B A B Solid Electrolyte Sheet 143 174 152 141 131 Thickness [μm]Solid Electrolyte Sheet 267 487 312 286 277 Flatness [μm] SolidElectrolyte Powder A A A B B Average Voltage [V] 2.5 — 2.3 2.4 2.1Discharge Capacity 52 — 41 49 38 [mAh/g] Rapid Charge/Discharge 46 — 3942 35 Char. [%] Volume Energy Density 32.0 — 22.2 29.2 16.7 [mWh/cm³]

(a-1) Production of Solid Electrolyte Sheet A Preparation of Slurry

Using sodium carbonate (Na₂CO₃), aluminum oxide (Al₂O₃), magnesium oxide(MgO), zirconium oxide (ZrO₂), and yttrium oxide (Y₂O₃) as rawmaterials, a raw material powder was prepared to have a composition of,in terms of % by mole, 14.2% Na₂O, 75.4% Al₂O₃, 5.4% MgO, 4.9% ZrO₂, and0.1% Y₂O₃. FIG. 2 shows a powder X-ray diffraction pattern of the rawmaterial powder. The powder X-ray diffraction pattern was measured usingan X-ray diffractometer (RINT-2000 manufactured by Rigaku Corporation).In Examples 1 to 3 and 5, a powder obtained by pre-firing the rawmaterial powder at 1250° C. for four hours, then grinding it, andclassifying the ground product was used for the preparation of a slurry.When the powder X-ray diffraction pattern of the raw material powderafter being pre-fired was checked, diffraction lines originating from ahexagonal crystal (β-alumina, i.e., (Al_(10.35)Mg_(0.65)O₁₆)(Na_(1.65)O)) belonging to space group P63 and diffraction linesoriginating from a trigonal crystal (β″-alumina, i.e.,(Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O)) belonging to space group R-3mwere confirmed. FIG. 3 shows a powder X-ray diffraction pattern of thepre-fired raw material powder. Furthermore, the rate of change to β″determined from the X-ray diffraction pattern was 54%. The rate ofchange to β″ was determined in the following manner.Rate of Change to β″=Iβ″/(Iβ+Iβ″)×100%

Iβ: peak intensity of β-alumina phase

Iβ″: peak intensity of β″-alumina phase

The peak intensity Iβ used was a value of 4.5 times the intensity of the(1, 0, 7) plane of the β-alumina phase and the peak intensity Iβ″ usedwas a value of 4.2 times the peak intensity of the (0, 2, 10) plane ofthe β″-alumina phase. In Comparative Examples 1, 2, and 4, the rawmaterial powder was used as it was, without pre-firing, for thepreparation of a slurry. Next, the powdered raw materials were wet mixedfor four hours using ethanol as a medium. After ethanol was evaporatedfrom the mixture, an acrylic acid ester-based copolymer (OLYCOX 1700manufactured by Kyoeisha Chemical Co., Ltd.) as a binder and benzylbutyl phthalate as a plasticizer were used and weighed with the mixtureto reach a ratio of powdered raw materials to binder to plasticizer of83.5:15:1.5 (mass ratio) and the mixture was dispersed intoN-methylpyrrolidinone, followed by well stirring with a planetarycentrifugal mixer to form a slurry.

Preparation of Green Sheet

The slurry obtained as above was applied onto a PET film using a doctorblade and dried at 70° C., thus obtaining a green sheet.

(Pressing and Firing of Green Sheet)

The obtained green sheet was cut into 15-mm squares and pressed at 90°C. and 40 MPa for five minutes using an isostatic pressing apparatus.The pressed green sheet was placed between an upper setter and a lowersetter (both formed of a MgO sheet), a 10 μm clearance was providedbetween the green sheet and the upper setter, and the green sheet inthis state was fired at 1600° C. for 30 minutes, thus obtaining a solidelectrolyte sheet A. When the powder X-ray diffraction pattern of thesolid electrolyte sheet A was checked, diffraction lines originatingfrom a hexagonal crystal (β-alumina, i.e., (Al_(10.35)Mg_(0.6)O₁₆)(Na_(1.65)O)) belonging to space group P63 and diffraction linesoriginating from a trigonal crystal (β″-alumina, i.e.,(Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O)) belonging to space group R-3mwere confirmed. FIG. 4 shows a powder X-ray diffraction pattern of thesolid electrolyte sheet A. The rate of change to β″ determined from theX-ray diffraction pattern was 82%.

(a-2) Production of Solid Electrolyte Sheet B Preparation of Slurry

Using sodium carbonate (Na₂CO₃), yttria-stabilized zirconia((ZrO₂)_(0.97)(Y₂O₃)_(0.03)), silicon dioxide (SiO₂), and sodiummetaphosphate (NaPO₃), a raw material powder was formulated to have acomposition of, in terms of % by mole, 25.3% Na₂O, 31.6% ZrO₂, 1% Y₂O₃,33.7% SiO₂, and 8.4% P₂O₅. In Examples 4 and 6, a powder obtained bypre-firing the raw material powder at 1100° C. for eight hours, thengrinding it, and classifying the ground product was used. In ComparativeExamples 3 and 5, the raw material powder was used as it was, withoutpre-firing. Next, the powdered raw materials were wet mixed for fourhours using ethanol as a medium. After ethanol was evaporated from themixture, an acrylic acid ester-based copolymer (OLYCOX 1700 manufacturedby Kyoeisha Chemical Co., Ltd.) as a binder and benzyl butyl phthalateas a plasticizer were used and weighed with the mixture to reach a ratioof powdered raw materials to binder to plasticizer of 83.5:15:1.5 (massratio) and the mixture was dispersed into N-methylpyrrolidinone,followed by well stirring with a planetary centrifugal mixer to form aslurry.

Preparation of Green Sheet

The slurry obtained as above was applied onto a PET film using a doctorblade and dried at 70° C., thus obtaining a green sheet.

(Pressing and Firing of Green Sheet)

The obtained green sheet was cut into 15-mm squares and pressed at 90°C. and 40 MPa for five minutes using an isostatic pressing apparatus.The pressed green sheet was placed between an upper setter and a lowersetter (both formed of a Pt sheet), a 10 μm clearance was providedbetween the green sheet and the upper setter, and the green sheet inthis state was fired at 1220° C. for 40 hours, thus obtaining a solidelectrolyte sheet B. When the powder X-ray diffraction pattern of thesolid electrolyte sheet B was checked, NASICON crystals were confirmed.

(b) Measurement of Flatness

The surface shape within a 10-mm square central portion of the obtainedsolid electrolyte sheet was measured with SURFCORDER ET4000AK(manufactured by Kosaka Laboratory Ltd.) under the conditions below. Thedifference between the maximum and minimum values of the height obtainedby the measurement was defined as a flatness.

X-direction measurement length: 10 mm

X-direction feed speed: 0.1 mm/s

Y-direction feed pitch: 200 μm

Number of Y-direction lines: 51

(c) Production of Sodium Ion All-Solid-State Secondary Battery (c-1)Preparation of Precursor Powder of Positive-Electrode Active MaterialCrystals

Using sodium metaphosphate (NaPO₃), ferric oxide (Fe₂O₃), andorthophosphoric acid (H₃PO₄) as raw materials, a raw material powder wasformulated to have a composition of, in % by mole, 40% Na₂O, 20% Fe₂O₃,and 40% P₂O₅. The raw material powder was melted in an air atmosphere at1250° C. for 45 minutes. Thereafter, the molten glass was poured betweena pair of rolls and formed into a film with rapid cooling, thuspreparing a precursor of positive-electrode active material crystals.

The obtained precursor of positive-electrode active material crystalswas ground for five hours in a ball mill using 20-mm diameter ZrO₂ ballsand the ground product was passed through a resin-made sieve with 120-μmopenings to obtain a coarse glass powder having an average particlediameter of 3 to 15 μm. Next, the coarse glass powder was ground, usingethanol as a grinding aid, for 80 hours in a ball mill using 3-mmdiameter ZrO₂ balls, thus obtaining a precursor powder ofpositive-electrode active material crystals having an average particlediameter of 0.7 μm.

To confirm precipitated active material crystals, 93% by mass precursorpowder of positive-electrode active material crystals obtained as aboveand 7% by mass acetylene black (SUPER C65 manufactured by TIMCAL) werewell mixed and the mixed powder was heat-treated at 450° C. for an hourin a mixed gas atmosphere of nitrogen and hydrogen (96% by volumenitrogen and 4% by volume hydrogen). When the powder X-ray diffractionpattern of the powder after being heat-treated was checked, diffractionlines originating from a triclinic crystal (Na₂FeP₂O₇) belonging tospace group P-1 were confirmed.

(c-2) Preparation of Solid Electrolyte Powder A

Using sodium carbonate (Na₂CO₃), aluminum oxide (Al₂O₃), magnesium oxide(MgO), zirconium oxide (ZrO₂), and yttrium oxide (Y₂O₃) as rawmaterials, a raw material powder was formulated to have a compositionof, in terms of % by mole, 14.2% Na₂O, 75.4% Al₂O₃, 5.4% MgO, 4.9% ZrO₂,and 0.1% Y₂O₃. The raw material powder was fired at 1250° C. for fourhours in an air atmosphere. The fired powder was ground for 24 hours ina ball mill using 20-mm diameter Al₂O₃ balls. Thereafter, the powder wasclassified by air to obtain a powder having an average particle diameterD50 of 2.0 μm. The obtained powder was formed into a 30-mm diametercylinder at a pressure of 11 MPa in a forming die, followed by heattreatment at 1600° C. for 30 minutes in an air atmosphere, thusobtaining sodium ion-conductive crystals (β″-alumina). The obtainedsodium ion-conductive crystals were ground with an alumina pestle in analumina mortar and the ground product was passed through a mesh with300-μm openings. The obtained powder was further ground, in a planetaryball mill P6 manufactured by Fritsch GmbH and loaded with 5-mm diameterZrO₂ balls, at 300 rpm for 30 minutes (with a 15-minute pause every 15minutes), and then passed through a mesh with 20-μm openings.Thereafter, the powder was classified with an air classifier, thusobtaining a solid electrolyte powder A containing β″-alumina. Thepreparation of sodium ion-conductive crystals and the preparation of asodium ion-conductive crystal powder were conducted in an environment ofthe dew point minus 40° C. or lower.

(c-3) Preparation of Solid Electrolyte Powder B

Using sodium carbonate (Na₂CO₃), yttria-stabilized zirconia((ZrO₂)_(0.97) (Y₂O₃)_(0.03)), silicon dioxide (SiO₂), and sodiummetaphosphate (NaPO₃), a raw material powder was formulated to have acomposition of, in terms of % by mole, 25.3% Na₂O, 31.6% ZrO₂, 1% Y₂O₃,33.7% SiO₂, and 8.4% P₂O₅. Next, the powdered raw materials were wetmixed for four hours using ethanol as a medium. Thereafter, ethanol wasevaporated, the powdered raw materials were pre-fired at 1100° C. foreight hours and then ground, and the ground powder was classified withan air classifier (type MDS-3 manufactured by Nippon Pneumatic Mfg. Co.,Ltd.). The classified powder was uniaxially pressed into shape at 11 MPain a 30-mm diameter die and then heat-treated at 1220° C. for 40 hoursto obtain a solid electrolyte containing sodium ion-conductive crystals(NASICON crystals).

The obtained solid electrolyte was ground with an alumina pestle in analumina mortar and the ground product was passed through a mesh with300-μm openings. The obtained powder was further ground, in a planetaryball mill P6 manufactured by Fritsch GmbH and loaded with 5-mm diameterZrO₂ balls, at 300 rpm for 30 minutes (with a 15-minute pause every 15minutes), and then passed through a mesh with 20-μm openings.Thereafter, the powder was classified with an air classifier, thusobtaining a solid electrolyte powder B containing sodium ion-conductivecrystals (NASICON crystals). The preparation of sodium ion-conductivecrystals and the preparation of a sodium ion-conductive crystal powderwere conducted in an environment of the dew point minus 40° C. or lower.

(c-4) Production of Test Cell

A precursor powder of positive-electrode active material crystals, asolid electrolyte powder A, and acetylene black (SUPER C65 manufacturedby TIMCAL) were weighed to reach, in terms of % by mass, 72%, 25%, and3%, respectively, and these powders were mixed for approximately 30minutes with an agate pestle in an agate mortar. An amount of 20 partsby mass of N-methylpyrrolidinone containing 10% by mass polypropylenecarbonate (manufactured by Sumitomo Seika Chemicals Co., Ltd.) was addedto 100 parts by mass of the mixed powder and the mixture was stirredwell with a planetary centrifugal mixer to form a slurry. All the aboveoperations were conducted in an environment of the dew point minus 40°C. or lower.

A masking tape having a 10-mm square opening and a thickness of 100 μmwas attached to the surface of each of the solid electrolyte sheets Ahaving a thickness shown in Table 1 or 2 and the slurry was applied tothe masking tape-attached solid electrolyte sheet A with a squeegee andthen dried at 70° C. for three hours. Next, the slurry on the solidelectrolyte sheet was fired at 450° C. for an hour in a mixed gasatmosphere of nitrogen and hydrogen (96% by volume nitrogen and 4% byvolume hydrogen), thus forming a positive electrode layer on the onesurface of the solid electrolyte sheet. When the X-ray diffractionpattern of the obtained positive electrode layer was checked,diffraction lines originating from a triclinic crystal (Na₂FeP₂O₇),which is an active material crystal and belongs to space group P-1,diffraction lines originating from a hexagonal crystal (β-alumina, i.e.,(Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O)), which is a sodium ion-conductivecrystal and belongs to space group P63, and diffraction linesoriginating from a trigonal crystal (β″-alumina, i.e.,(Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O)) belonging to space group R-3mwere confirmed.

A current collector of a 300-nm thick gold electrode was formed on thesurface of the positive electrode layer using a sputtering device(SC-701AT manufactured by Sanyu Electron Co., Ltd.). Thereafter,metallic sodium serving as a counter electrode was pressure-bonded tothe other surface of the solid electrolyte sheet opposite to the surfacethereof on which the positive electrode layer was formed, in an argonatmosphere of the dew point minus 60° C. or lower, and the obtainedproduct was placed on a lower lid of a coin cell and covered with anupper lid to produce a CR2032-type test cell.

In Example 4 and Comparative Example 3, a test cell was produced in thesame manner as described above except that the solid electrolyte sheet Bhaving a thickness shown in Table 1 or 2 was used instead of the solidelectrolyte sheet A. In Example 5 and Comparative Example 4, a test cellwas produced in the same manner as described above except that the solidelectrolyte powder B was used instead of the solid electrolyte powder A.In Example 6 and Comparative Example 5, a test cell was produced in thesame manner as described above except that the solid electrolyte sheet Bhaving a thickness shown in Table 1 or 2 was used instead of the solidelectrolyte sheet A and the solid electrolyte powder B was used insteadof the solid electrolyte powder A.

(c-5) Charge and Discharge Test

A charge and discharge test was conducted at 30° C. using each of theobtained test cells to measure the discharge capacity. The results areshown in Tables 1 and 2. In the charge and discharge test, charging(release of sodium ions from the positive-electrode active material) wasimplemented by CC (constant-current) charging from the open circuitvoltage (OCV) to 4.3 V and discharging (absorption of sodium ions to thepositive-electrode active material) was implemented by CC dischargingfrom 4.3 V to 2 V. The C rate was 0.01 C.

The average voltage and discharge capacity in Tables 1 and 2 mean thefirst average voltage and first discharge capacity evaluated at a C rateof 0.01 C and the rapid charge/discharge characteristic in Tables 1 and2 indicates the ratio of the first discharge capacity at a C rate of 0.1C to the first discharge capacity at a C rate of 0.01 C ((0.1 C/0.01C)×100(%)).

Examples 1 to 6 exhibited an average voltage of 2.5 to 2.8 V, adischarge capacity of 61 to 73 mAh/g, a rapid charge/dischargecharacteristic of 64 to 82%, and an energy density of 37.4 to 82.3mWh/cm³, and were therefore excellent in these characteristics. On theother hand, Comparative Examples 1 and 3 to 5 exhibited an averagevoltage of 2.5 V or less, a discharge capacity of 52 mAh/g or less, arapid charge/discharge characteristic of 46% or less, and an energydensity of 32.0 mWh/cm³ or less, and were therefore poor in thesecharacteristics. In Comparative Example 2 in which the flatness of thesolid electrolyte sheet was as significantly large as 487 μm, the solidelectrolyte sheet was broken during production of a cell and, therefore,the cell characteristics could not be measured.

Solid Electrolyte Sheet According to Second Aspect

Table 3 shows Examples 1 to 6 and Table 4 shows Comparative Examples 1to 5.

TABLE 3 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Pre-firing done done donedone done done Solid Electrolyte Sheet A A A B A B Solid ElectrolyteSheet 67 103 448 54 62 74 Thickness [μm] Na₂O Concentration C₁ at 12.49.8 11.4 11.5 10.6 11.6 Depth of 5 μm from Surface [% by mole] Na₂OConcentration C₂ at 12.8 11.2 12.1 12.4 11.9 12.1 Depth of 20 μm fromSurface [% by mole] C₂ − C₁ [% by mole] 0.4 1.4 0.7 0.9 1.3 0.5 Na₂OConcentration C₁′ at 12.3 9.9 12.2 11.2 10.5 11.3 Depth of 5% fromSurface [% by mole] Na₂O Concentration C₂′ at 12.8 11.4 12.4 12.5 12.112.2 Depth of 50% from Surface [% by mole] C₂′ − C₁′ [% by mole] 0.5 1.50.2 1.3 1.6 0.9 Solid Electrolyte Powder A A A A B B Ionic Conductivity5.1 4.6 3.3 0.3 4.8 0.5 [×10⁻³ S/cm] Discharge Capacity [mAh/g] 47 43 4034 37 35 Volume Energy Density 63.5 45.3 13.5 33.3 33.9 29.2 [mWh/cm³]

TABLE 4 Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5Pre-firing no no no no no Solid Electrolyte Sheet A A B A B SolidElectrolyte Sheet 73 59 63 71 65 Thickness [μm] Na₂O Concentration C₁1.2 0.1 0.8 1.3 0.5 at Depth of 5 μm from Surface [% by mole] Na₂OConcentration C₂ 12.7 11.3 12.4 11.9 11.2 at Depth of 20 μm from Surface[% by mole] C₂ − C₁ [% by mole] 11.5 11.2 11.6 10.6 10.7 Na₂OConcentration C₁′ 0.9 0.1 0.4 1.1 0.3 at Depth of 5% from Surface [% bymole] Na₂O Concentration C₂′ 12.8 11.5 12.5 12.2 11.6 at Depth of 50%from Surface [% by mole] C₂′ − C₁′ 11.9 11.4 12.1 11.1 11.3 [% by mole]Solid Electrolyte Powder A A A B B Ionic Conductivity 0.08 0.003 0.00040.007 0.0001 [×10⁻³ S/cm] Discharge Capacity 7 2 1 3 2 [mAh/g] VolumeEnergy 9.0 2.9 0.9 2.6 1.8 Density [mWh/cm³]

(a-1) Production of Solid Electrolyte Sheet A Preparation of Slurry

Using sodium carbonate (Na₂CO₃), aluminum oxide (Al₂O₃), magnesium oxide(MgO), zirconium oxide (ZrO₂), and yttrium oxide (Y₂O₃) as rawmaterials, a raw material powder was prepared to have a composition of,in terms of % by mole, 14.2% Na₂O, 75.4% Al₂O₃, 5.4% MgO, 4.9% ZrO₂, and0.1% Y₂O₃.

FIG. 5 shows a powder X-ray diffraction pattern of the raw materialpowder. The powder X-ray diffraction pattern was measured using an X-raydiffractometer (RINT-2000 manufactured by Rigaku Corporation). InExamples 1 to 3 and 5, a powder obtained by pre-firing the raw materialpowder at 1250° C. for four hours, then grinding it, and classifying theground product was used for the preparation of a slurry. When the powderX-ray diffraction pattern of the raw material powder after beingpre-fired was checked, diffraction lines originating from a hexagonalcrystal (β-alumina, i.e., (Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O))belonging to space group P63 and diffraction lines originating from atrigonal crystal (β″-alumina, i.e., (Al_(10.35)Mg_(0.65)O₁₆)(Na_(1.65)O)) belonging to space group R-3m were confirmed. FIG. 6 showsa powder X-ray diffraction pattern of the raw material powder afterbeing pre-fired. Furthermore, the rate of change to β″ determined fromthe X-ray diffraction pattern was 54%. The rate of change to β″ wasdetermined in the manner as described previously. In ComparativeExamples 1, 2, and 4, the raw material powder was used as it was,without pre-firing, for the preparation of a slurry.

Next, the powdered raw materials were wet mixed for four hours usingethanol as a medium. After ethanol was evaporated from the mixture, anacrylic acid ester-based copolymer (OLYCOX 1700 manufactured by KyoeishaChemical Co., Ltd.) as a binder and benzyl butyl phthalate as aplasticizer were used and weighed with the mixture to reach a ratio ofpowdered raw materials to binder to plasticizer of 83.5:15:1.5 (massratio) and the mixture was dispersed into N-methylpyrrolidinone,followed by well stirring with a planetary centrifugal mixer to form aslurry.

Preparation of Green Sheet

The slurry obtained as above was applied onto a PET film using a doctorblade and dried at 70° C., thus obtaining a green sheet.

(Pressing and Firing of Green Sheet)

The obtained green sheet was cut into 15-mm squares and pressed at 90°C. and 40 MPa for five minutes using an isostatic pressing apparatus.The pressed green sheet was placed between an upper setter and a lowersetter (both formed of a MgO sheet), a 10 μm clearance was providedbetween the green sheet and the upper setter, and the green sheet inthis state was fired at 1600° C. for 30 minutes, thus obtaining a solidelectrolyte sheet A. When the powder X-ray diffraction pattern of thesolid electrolyte sheet A was checked, diffraction lines originatingfrom a hexagonal crystal (β-alumina, i.e., (Al_(10.35)Mg_(0.65)O₁₆)(Na_(1.65)O)) belonging to space group P63 and diffraction linesoriginating from a trigonal crystal (β″-alumina, i.e.,(Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O)) belonging to space group R-3mwere confirmed. FIG. 7 shows a powder X-ray diffraction pattern of thesolid electrolyte sheet A. The rate of change to β″ determined from theX-ray diffraction pattern was 82%.

(a-2) Production of Solid Electrolyte Sheet B Preparation of Slurry

Using sodium carbonate (Na₂CO₃), yttria-stabilized zirconia((ZrO₂)_(0.97)(Y₂O₃)_(0.03)), silicon dioxide (SiO₂), and sodiummetaphosphate (NaPO₃), a raw material powder was formulated to have acomposition of, in terms of % by mole, 25.3% Na₂O, 31.6% ZrO₂, 1% Y₂O₃,33.7% SiO₂, and 8.4% P₂O₅. In Examples 4 and 6, a powder obtained bypre-firing the raw material powder at 1100° C. for eight hours, thengrinding it, and classifying the ground product was used. In ComparativeExamples 3 and 5, the raw material powder was used as it was, withoutpre-firing. Next, the powdered raw materials were wet mixed for fourhours using ethanol as a medium. After ethanol was evaporated from themixture, an acrylic acid ester-based copolymer (OLYCOX 1700 manufacturedby Kyoeisha Chemical Co., Ltd.) as a binder and benzyl butyl phthalateas a plasticizer were used and weighed with the mixture to reach a ratioof powdered raw materials to binder to plasticizer of 83.5:15:1.5 (massratio) and the mixture was dispersed into N-methylpyrrolidinone,followed by well stirring with a planetary centrifugal mixer to form aslurry.

Preparation of Green Sheet

The slurry obtained as above was applied onto a PET film using a doctorblade and dried at 70° C., thus obtaining a green sheet.

(Pressing and Firing of Green Sheet)

The obtained green sheet was cut into 15-mm squares and pressed at 90°C. and 40 MPa for five minutes using an isostatic pressing apparatus.The pressed green sheet was placed between an upper setter and a lowersetter (both formed of a Pt sheet), a 10 μm clearance was providedbetween the green sheet and the upper setter, and the green sheet inthis state was fired at 1220° C. for 40 hours, thus obtaining a solidelectrolyte sheet B. When the powder X-ray diffraction pattern of thesolid electrolyte sheet B was checked, NASICON crystals were confirmed.

(b) Measurement of Surface Layer Na₂O Concentration

A cut surface of each of the solid electrolyte sheets was measured byEDX (energy dispersive analysis by X-ray) to obtain the respective Na₂Oconcentrations at depths of 5 μm and 20 μm from the surface of the solidelectrolyte sheet.

(c) Measurement of Ionic Conductivity

After a gold electrode was formed as an ion blocking electrode on asurface of the solid electrolyte sheet, the electrode was measuredwithin a frequency range of 1 to 107 Hz by the AC impedance method toobtain a resistance value from a Cole-Cole plot. An ionic conductivitywas calculated from the obtained resistance value. The measurement wasconducted at 25° C.

(d) Production of Sodium Ion All-Solid-State Secondary Battery (d-1)Preparation of Precursor Powder of Positive-Electrode Active MaterialCrystals

Using sodium metaphosphate (NaPO₃), ferric oxide (Fe₂O₃), andorthophosphoric acid (H₃PO₄) as raw materials, a raw material powder wasformulated to have a composition of, in % by mole, 40% Na₂O, 20% Fe₂O₃,and 40% P₂O₅. The raw material powder was melted in an air atmosphere at1250° C. for 45 minutes. Thereafter, the molten glass was poured betweena pair of rolls and formed into a film with rapid cooling, thuspreparing a precursor of positive-electrode active material crystals.

The obtained precursor of positive-electrode active material crystalswas ground for five hours in a ball mill using 20-mm diameter ZrO₂ ballsand the ground product was passed through a resin-made sieve with 120-μmopenings to obtain a coarse glass powder having an average particlediameter of 3 to 15 μm. Next, the coarse glass powder was ground, usingethanol as a grinding aid, for 80 hours in a ball mill using 3-mmdiameter ZrO₂ balls, thus obtaining a precursor powder ofpositive-electrode active material crystals having an average particlediameter of 0.7 μm.

To confirm precipitated active material crystals, 93% by mass precursorpowder of positive-electrode active material crystals obtained as aboveand 7% by mass acetylene black (SUPER C65 manufactured by TIMCAL) werewell mixed and the mixed powder was heat-treated at 450° C. for an hourin a mixed gas atmosphere of nitrogen and hydrogen (96% by volumenitrogen and 4% by volume hydrogen). When the powder X-ray diffractionpattern of the powder after being heat-treated was checked, diffractionlines originating from a triclinic crystal (Na₂FeP₂O₇) belonging tospace group P-1 were confirmed.

(d-2) Preparation of Solid Electrolyte Powder A

Using sodium carbonate (Na₂CO₃), aluminum oxide (Al₂O₃), magnesium oxide(MgO), zirconium oxide (ZrO₂), and yttrium oxide (Y₂O₃) as rawmaterials, a raw material powder was formulated to have a compositionof, in terms of % by mole, 14.2% Na₂O, 75.4% Al₂O₃, 5.4% MgO, 4.9% ZrO₂,and 0.1% Y₂O₃. The raw material powder was fired at 1250° C. for fourhours in an air atmosphere. The fired powder was ground for 24 hours ina ball mill using 20-mm diameter Al₂O₃ balls. Thereafter, the powder wasclassified by air to obtain a powder having an average particle diameterD50 of 2.0 μm. The obtained powder was formed into a 30-mm diametercylinder at a pressure of 11 MPa in a forming die, followed by heattreatment at 1600° C. for 30 minutes in an air atmosphere, thusobtaining sodium ion-conductive crystals (β″-alumina). The obtainedsodium ion-conductive crystals were ground with an alumina pestle in analumina mortar and the ground product was passed through a mesh with300-μm openings. The obtained powder was further ground, in a planetaryball mill P6 manufactured by Fritsch GmbH and loaded with 5-mm diameterZrO₂ balls, at 300 rpm for 30 minutes (with a 15-minute pause every 15minutes), and then passed through a mesh with 20-μm openings.Thereafter, the powder was classified with an air classifier, thusobtaining a solid electrolyte powder A containing β″-alumina. Thepreparation of sodium ion-conductive crystals and the preparation of asodium ion-conductive crystal powder were conducted in an environment ofthe dew point minus 40° C. or lower.

(d-3) Preparation of Solid Electrolyte Powder B

Using sodium carbonate (Na₂CO₃), yttria-stabilized zirconia((ZrO₂)_(0.97)(Y₂O₃)_(0.03)), silicon dioxide (SiO₂), and sodiummetaphosphate (NaPO₃), a raw material powder was formulated to have acomposition of, in terms of % by mole, 25.3% Na₂O, 31.6% ZrO₂, 1% Y₂O₃,33.7% SiO₂, and 8.4% P₂O₅. Next, the powdered raw materials were wetmixed for four hours using ethanol as a medium. Thereafter, ethanol wasevaporated, the powdered raw materials were pre-fired at 1100° C. foreight hours and then ground, and the ground powder was classified withan air classifier (type MDS-3 manufactured by Nippon Pneumatic Mfg. Co.,Ltd.). The classified powder was uniaxially pressed into shape at 11 MPain a 30-mm diameter die and then heat-treated at 1220° C. for 40 hoursto obtain a solid electrolyte containing sodium ion-conductive crystals(NASICON crystals).

The obtained solid electrolyte was ground with an alumina pestle in analumina mortar and the ground product was passed through a mesh with300-μm openings. The obtained powder was further ground, in a planetaryball mill P6 manufactured by Fritsch GmbH and loaded with 5-mm diameterZrO₂ balls, at 300 rpm for 30 minutes (with a 15-minute pause every 15minutes), and then passed through a mesh with 20-μm openings.Thereafter, the powder was classified with an air classifier, thusobtaining a solid electrolyte powder B containing sodium ion-conductivecrystals (NASICON crystals). The preparation of sodium ion-conductivecrystals and the preparation of a sodium ion-conductive crystal powderwere conducted in an environment of the dew point minus 40° C. or lower.

(d-4) Production of Test Cell

A precursor powder of positive-electrode active material crystals, asolid electrolyte powder A, and acetylene black (SUPER C65 manufacturedby TIMCAL) were weighed to reach, in terms of % by mass, 72%, 25%, and3%, respectively, and these powders were mixed for approximately 30minutes with an agate pestle in an agate mortar. An amount of 20 partsby mass of N-methylpyrrolidinone containing 10% by mass polypropylenecarbonate (manufactured by Sumitomo Seika Chemicals Co., Ltd.) was addedto 100 parts by mass of the mixed powder and the mixture was stirredwell with a planetary centrifugal mixer to form a slurry. All the aboveoperations were conducted in an environment of the dew point minus 40°C. or lower.

A masking tape having a 10-mm square opening and a thickness of 100 μmwas attached to the surface of each of the solid electrolyte sheets Ahaving a thickness shown in Table 3 or 4 and the slurry was applied tothe masking tape-attached solid electrolyte sheet A with a squeegee andthen dried at 70° C. for three hours. Next, the slurry on the solidelectrolyte sheet was fired at 450° C. for an hour in a mixed gasatmosphere of nitrogen and hydrogen (96% by volume nitrogen and 4% byvolume hydrogen), thus forming a positive electrode layer on the onesurface of the solid electrolyte sheet. When the X-ray diffractionpattern of the obtained positive electrode layer was checked,diffraction lines originating from a triclinic crystal (Na₂FeP₂O₇),which is an active material crystal and belongs to space group P-1,diffraction lines originating from a hexagonal crystal (β-alumina, i.e.,(Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O)), which is a sodium ion-conductivecrystal and belongs to space group P63, and diffraction linesoriginating from a trigonal crystal (β″-alumina, i.e.,(Al_(10.35)Mg_(0.65)O₁₆) (Na_(1.65)O)) belonging to space group R-3mwere confirmed.

A current collector of a 300-nm thick gold electrode was formed on thesurface of the positive electrode layer using a sputtering device(SC-701AT manufactured by Sanyu Electron Co., Ltd.). Thereafter,metallic sodium serving as a counter electrode was pressure-bonded tothe other surface of the solid electrolyte sheet opposite to the surfacethereof on which the positive electrode layer was formed, in an argonatmosphere of the dew point minus 60° C. or lower, and the obtainedproduct was placed on a lower lid of a coin cell and covered with anupper lid to produce a CR2032-type test cell.

In Example 4 and Comparative Example 3, a test cell was produced in thesame manner as described above except that the solid electrolyte sheet Bhaving a thickness shown in Table 3 or 4 was used instead of the solidelectrolyte sheet A. In Example 5 and Comparative Example 4, a test cellwas produced in the same manner as described above except that the solidelectrolyte powder B was used instead of the solid electrolyte powder A.In Example 6 and Comparative Example 5, a test cell was produced in thesame manner as described above except that the solid electrolyte sheet Bhaving a thickness shown in Table 3 or 4 was used instead of the solidelectrolyte sheet A and the solid electrolyte powder B was used insteadof the solid electrolyte powder A.

(d-5) Charge and Discharge Test

A charge and discharge test was conducted at 30° C. using each of theobtained test cells to measure the discharge capacity. The results areshown in Tables 3 and 4. In the charge and discharge test, charging(release of sodium ions from the positive-electrode active material) wasimplemented by CC (constant-current) charging from the open circuitvoltage (OCV) to 4.3 V and discharging (absorption of sodium ions to thepositive-electrode active material) was implemented by CC dischargingfrom 4.3 V to 2 V. The C rate was 0.01 C. The discharge capacity inTables 3 and 4 means the first discharge capacity evaluated at a C rateof 0.01 C.

In Examples 1 to 6, the difference C₂−C₁ between the Na₂O concentrationC₁ at a depth of 5 μm from a surface of the solid electrolyte sheet andthe Na₂O concentration C₂ at a depth of 20 μm from the surface of thesolid electrolyte sheet was as small as 0.4 to 1.4% by mole, and, withthe thickness of the solid electrolyte sheet as 100%, the differenceC₂′−C₁′ between the Na₂O concentration C₁′ at a depth of 5% from thesurface of the solid electrolyte sheet and the Na₂O concentration C₂′ ata depth of 50% from the surface of the solid electrolyte sheet was assmall as 0.2 to 1.6% by mole. Therefore, the ionic conductivity was aslarge as 0.3×10⁻³ to 5.1×10⁻³ S/cm. Hence, in the cells in which theabove solid electrolytes were used, their discharge capacity was aslarge as 34 to 47 mAh/g and their volume energy density was as large as13.5 to 63.5 mWh/cm³.

On the other hand, in Comparative Examples 1 to 5, the difference C₂−C₁between the Na₂O concentration C₁ at a depth of 5 μm from a surface ofthe solid electrolyte sheet and the Na₂O concentration C₂ at a depth of20 μm from the surface of the solid electrolyte sheet was as large as10.6 to 11.6% by mole, and, with the thickness of the solid electrolytesheet as 100%, the difference C₂′−C₁′ between the Na₂O concentration C₁′at a depth of 5% from the surface of the solid electrolyte sheet and theNa₂O concentration C₂′ at a depth of 50% from the surface of the solidelectrolyte sheet was as large as 11.1 to 12.1% by mole. Therefore, theionic conductivity was as small as 0.0001×10⁻³ to 0.08×10⁻³ S/cm. Hence,in the cells in which the above solid electrolytes were used, theirdischarge capacity was as small as 1 to 7 mAh/g and their volume energydensity was as small as 0.9 to 9.0 mWh/cm³.

INDUSTRIAL APPLICABILITY

The solid electrolyte sheet according to the present invention issuitable not only for use in batteries, including a sodium ionall-solid-state secondary battery and a sodium-sulfur battery, but alsoas solid electrolytes for gas sensors, including a CO₂ sensor and an NO₂sensor.

The invention claimed is:
 1. A solid electrolyte sheet with an electrodecomprising: a solid electrolyte sheet containing at least one type ofsodium ion-conductive crystal selected from β″-alumina and NASICONcrystal, the solid electrolyte sheet having a thickness of 500 μm orless and a flatness of 200 μm or less; and a positive electrode layer ora negative electrode layer formed on one surface of the solidelectrolyte sheet.
 2. The solid electrolyte sheet with an electrodeaccording to claim 1, further comprising a current collector formed onthe surface opposite to the solid electrolyte sheet side of the positiveelectrode layer or the negative electrode layer.
 3. The solidelectrolyte sheet with an electrode according to claim 1, the solidelectrolyte sheet containing, in terms of % by mole, 65 to 95% Al₂O₃, 2to 20% Na₂O, 0.3 to 15% MgO+Li₂O, 0 to 20% ZrO₂, and 0 to 5% Y₂O₃. 4.The solid electrolyte sheet with an electrode according to claim 1, thesolid electrolyte sheet containing crystals represented by a generalformula Na_(s)A1_(t)A2_(u)O_(v) (where A1 is at least one selected fromAl, Y, Yb, Nd, Nb, Ti, Hf, and Zr, A2 is at least one selected from Siand P, s=1.4 to 5.2, t=1 to 2.9, u=2.8 to 4.1, and v=9 to 14).
 5. Thesolid electrolyte sheet with an electrode according to claim 1, for usein an all-solid-state sodium ion secondary battery.
 6. A solidelectrolyte sheet with an electrode comprising: a solid electrolytesheet containing at least one type of sodium ion-conductive crystalselected from β″-alumina and NASICON crystal, wherein when C₁ representsa Na₂O concentration at a depth of 5 μm from a surface of the solidelectrolyte sheet and C₂ represents a Na₂O concentration at a depth of20 μm from the surface, C₂−C₁≤10% by mole; and a positive electrodelayer or a negative electrode layer formed on one surface of the solidelectrolyte sheet.
 7. The solid electrolyte sheet with an electrodeaccording to claim 6, further comprising a current collector formed onthe surface opposite to the solid electrolyte sheet side of the positiveelectrode layer or the negative electrode layer.
 8. The solidelectrolyte sheet with an electrode according to claim 6, having athickness of 500 μm or less.
 9. The solid electrolyte sheet with anelectrode according to claim 6, the solid electrolyte sheet containing,in terms of % by mole, 65 to 95% Al₂O₃, 2 to 20% Na₂O, 0.3 to 15%MgO+Li₂O, 0 to 20% ZrO₂, and 0 to 5% Y₂O₃.
 10. The solid electrolytesheet with an electrode according to claim 6, the solid electrolytesheet containing crystals represented by a general formulaNa_(s)A1_(t)A2_(u)O_(v) (where A1 is at least one selected from Al, Y,Yb, Nd, Nb, Ti, Hf, and Zr, A2 is at least one selected from Si and P,s=1.4 to 5.2, t=1 to 2.9, u=2.8 to 4.1, and v=9 to 14).
 11. The solidelectrolyte sheet with an electrode according to claim 6, for use in anall-solid-state sodium ion secondary battery.
 12. A solid electrolytesheet with an electrode comprising: a solid electrolyte sheet containingat least one type of sodium ion-conductive crystal selected fromβ″-alumina and NASICON crystal, wherein when, with a thickness of thesolid electrolyte sheet represented as 100%, C₁′ represents a Na₂Oconcentration at a depth of 5% from a surface of the solid electrolytesheet and C₂′ represents a Na₂O concentration at a depth of 50% from thesurface, C₂′−C₁′≤10% by mole; and a positive electrode layer or anegative electrode layer formed on one surface of the solid electrolytesheet.
 13. The solid electrolyte sheet with an electrode according toclaim 12, further comprising a current collector formed on the surfaceopposite to the solid electrolyte sheet side of the positive electrodelayer or the negative electrode layer.
 14. The solid electrolyte sheetwith an electrode according to claim 12, having a thickness of 500 μm orless.
 15. The solid electrolyte sheet with an electrode according toclaim 12, the solid electrolyte sheet containing, in terms of % by mole,65 to 95% Al₂O₃, 2 to 20% Na₂O, 0.3 to 15% MgO+Li₂O, 0 to 20% ZrO₂, and0 to 5% Y₂O₃.
 16. The solid electrolyte sheet with an electrodeaccording to claim 12, the solid electrolyte sheet containing crystalsrepresented by a general formula Na_(s)A1_(t)A2_(u)O_(v) (where A1 is atleast one selected from Al, Y, Yb, Nd, Nb, Ti, Hf, and Zr, A2 is atleast one selected from Si and P, s=1.4 to 5.2, t=1 to 2.9, u=2.8 to4.1, and v=9 to 14).
 17. The solid electrolyte sheet with an electrodeaccording to claim 12, for use in an all-solid-state sodium ionsecondary battery.